The present invention is generally directed to Solid Oxide Fuel Cells (SOFC's), and more specifically to reversible SOFC's referred to as Solid Oxide Regenerative Fuel Cells (SORFC's).
The overall potential efficiency of the SORFC is restrained by a charging voltage for very high efficiency which is below the thermal neutral voltage. This means that heat must be added to the SORFC operating in the charge or electrolysis mode in order to keep it at operational temperature to operate at these voltage levels.
There is an abundance of extra heat generated during the SORFC discharge or fuel cell mode. One method of obtaining a high SORFC round trip efficiency is to store the extra heat produced in the discharge mode and use that heat to maintain the system temperature during the charge mode. This requires an appropriate high heat capacity material to accomplish adequate heat storage. Such a heat storage system is appropriate for a very high efficiency SORFC based on a water cycle. In such a cycle, water is electrolyzed during the charge or electrolysis mode with the product hydrogen stored and the product oxygen discharged to ambient. During the discharge or fuel cell mode of the SORFC based on the water cycle, the hydrogen and oxygen from air are reacted to produce power and water. However, the heat storage material increases system mass and complexity, which may be disadvantageous for certain applications.